Military RF systems conventionally include wide bandwidth, high resolution Analog-to-Digital Converters (ADC) to enable, for example, wideband staring Signals Intelligence (SIGINT) receivers, flexible Software Defined Radio system architectures, and Low Probability of Intercept/Low Probability of Detection (LPI/LPD) radars. These systems often use electronic ADC (eADC) components.
Analog-to-Digital Converters (ADC) using optical or photonic ADCs (pADCs) are also known. For example, U.S. Pat. No. 8,446,305 entitled “PHOTONIC ANALOG TO DIGITAL CONVERSION” uses pADCs in its design. Typically in such pADC systems the IQ demodulator is photonic and a single phase modulator is used. The pADC samples an RF signal with a stream of optical pulses emitted by an optical pulse source.
A reference signal and the phase modulated signal are provided as optical signals to an optical IQ demodulator of the pADC. The optical IQ demodulator optically demodulates the reference signal and the phase modulated signal to provide optically demodulated in-phase (I) and quadrature-phase (Q) signals. The optically demodulated I and Q signals are then sent to, and detected by, photodetectors, the electronic signals thereof are applied to a digitizer platform.
The pADC with a single phase modulator and the optical IQ demodulator has problems, however. The single phase modulator used in such signal/reference architecture has a relatively increased maximum modulator drive voltage. This increases the power consumption of the modulator drive circuit. In addition, the optical IQ demodulator adds to the optical propagation loss. Increased optical loss leads to increased required optical pulse power, and hence, electrical power dissipation. Further IQ imbalance is not tuned in the electrical domain.